1. Field of the Invention
This invention relates to a quadruple mode bandpass filter and, in particular, to a filter that has at least one cavity resonating in four independent orthogonal modes simultaneously.
2. Description of the Prior Art
It is known to have single, dual and triple mode bandpass filters. At the present time in the satellite communications industry, it is common to use 4-pole or 6-pole dual mode filters in output multiplexers and 8-pole dual mode filters in input multiplexers. Thus, by this arrangement, output multiplexers have either two or three cascade waveguide cavities and input multiplexers have four cascade waveguide cavities. In the satellite communications industry, any weight or volume savings achieved are extremely important. Filters currently used for input and output multiplexers are generally significantly heavier and occupy a much larger volume than filters made in accordance with the present invention. Further, it is known to use two triple mode cavities as a 6-pole filter in an output multiplexer. Unfortunately, this type of 6-pole filter must be launched onto a manifold of the multiplexer at a sidewall of one of the triple mode cavities. This side-wall launching can be much bulkier than an end-wall launching.
For some time, it has been known that if a filter can be made to produce more transmission zeros, the response of that filter will be enhanced. With previous filters, the maximum number of transmission zeros that can be produced is equal to the order of the filter minus two. For example, a six-pole prior art dual mode filter can be made to produce four transmission zeros and such a filter is said to produce an elliptic function response.
A bandpass filter having at least one four mode cavity, with tuning screws and coupling screws arranged in said filter so that said four mode cavity resonates at its resonant frequency in four independent orthogonal modes simultaneously. Any additional cavities to said four mode cavity are immediately adjacent to one another or to said four mode cavity. At least one of any additional cavities in immediately adjacent to said four mode cavity. Any immediately adjacent cavities are coupled to one another and resonating at their resonant frequency, said fliter having an input and output.
Preferably, the filter of the present invention has at least two cavities, a first cavity being a quadruple mode cavity and a second cavity being either a single mode cavity, a dual mode cavity, a triple mode cavity or a quadruple mode cavity. Still more preferably, the filter of the present invention is operated in such a manner that the number of transmission zeros is equal to the order of the filter.